1. Field of the Invention
The present invention is directed to a photocurable composition for use as an encapsulant, underfill or attachment adhesive, capable of curing at wavelengths greater than 290 nm. Reaction products of these photocurable compositions have a low level of extractable halide ion, such as less than 100 ppm and oftentimes less than 70 ppm. In use, the photocurable composition may be applied, for instance, over the wire bonds that electrically connect a semiconductor device to a substrate to maintain a fixed positional relationship and protect the integrity of the electrical connection from vibrational and shock disturbances, as well as from interference from environmental contaminants.
2. Brief Description of Related Technology
An integrated circuit assembly typically includes a substrate that forms a physical and structural foundation for an integrated circuit die, which itself is electrically connected to bonding pads on the substrate so as to allow for communication between the die and external devices.
In a smart card application, an integrated circuit module is disposed in a plastic card, similar in size and shape to an ordinary credit or debit card. The integrated circuit module itself includes a microcontroller and a memory device, such as an EPROM, of sufficient size to store large amounts of information, such as personal information, medical history information, financial information, security information, frequent flyer mileage information, and the like. The smart card can also operate as a telephone card with a stored monetary value that is updated with each transaction.
Heretofore, smart cards have included a plastic card, in which is milled out a pocket or cavity dimensioned and disposed to receive an integrated circuit module. The integrated circuit module is constructed of a die disposed over a substrate and electrically connected with gold or aluminum wire bonds. The wire bonds typically rise vertically from the die and bend over to a bonding pad on the substrate. (See FIG. 1.)
In order to protect the wire bonds from fracture or environmental damage, an encapsulant is ordinarily applied thereover. The encapsulant acts to seal in place the wire bonds, providing cushioning against shock (which may lead to fracture) and prevents the ingress of environmental contaminants which may cause an electrical disconnect.
One way to apply such an encapsulant during smart card assembly is by the so-called two-step “dam and fill” approach. Here, a dam is first cast from a highly thickened version of a lower-viscosity encapsulant, creating a wall around the device to be encapsulated. Alternatively, the device may be framed with a premolded wall or simply contained in a cavity that has been machined or fabricated into the board. See e.g., M. M. Konarski and J. Heaton, “Electronic Packaging Design Advances Miniaturization”, Circ. Assembly, 32-35 (August 1996).
The encapsulant is ordinarily designed to have high flow characteristics, and through high filler loadings, to match to a large degree the coefficients of thermal expansion (“CTE”) of the carrier substrate and the semiconductor device to which it is attached, by electrically-connecting wire bonds. The high flow characteristics—low viscosity—allows for easy penetration in and around ultrafine-pitch wire bond connection.
Once cured, these encapsulants should provide a high level of device reliability.
Early smart card encapsulants had been rendered curable through heat cure mechanisms. That is, after application of the encapsulant composition, cure would occur through exposure to elevated temperature conditions, such as about 100° C., for periods of time, such as about 16 hours. This would be ordinarily achieved through passage on line into a heating chamber. Products of this kind have been available commercially from Dexter Electronic Materials, City of Industry, California.
U.S. Pat. No. 5,863,970 (Ghoshal) speaks to heat curable epoxy resins of siloxane-based and non-siloxane-based epoxy resins, a filler, an iodonium salt cure catalyst and co-cure catalyst of a copper compound for use in electronic device assembly, such as die attach adhesives and encapsulants like underfill and glob top applications. More specifically, Ghoshal claims a filled resin composition whose components require a cycloalphatic epoxy-functional siloxane, a di-epoxy such as bis(3,4-epoxycyclohexyl) adipate, an iodonium salt whose counter ion may be pentafluoratetraphenyl borate, B(C6F5)4, an optional copper compound, to lower the temperature required to thermally cure the composition, a silane-based adhesion promoter and a polybutadiene elastomeric toughener.
Drawbacks exist to such heat-cure encapsulants, though. For instance, the heat cure step may compromise the integrity of the overall electronic device and/or substrate by virtue of the thermal exposure. In addition, the heat cure step presents a discontinuity and drag in the manufacturing throughput, particularly in view of the heating and cooling required, as well as the time required to cure the encapsulant under such heated conditions. Moreover, energy and labor requirements involved in the heat cure step add cost to the assembly of the electronic device, such as the smart card.
As a result, there has been an on-going desire to find photocurable encapsulants, because photocure mechanisms are ordinarily more rapid than heat cure mechanisms, and occur without exposure to the heat applied during heat cure. Thus, use of such photocurable encapsulants minimizes the tendency to compromise the integrity of the overall electronic device and/or substrate.
Photocurable encapsulants, even for smart card applications, are not themselves new. However, the earlier developments of photocurable smart card encapsulants presented drawbacks—for instance, the high level of halide ion, such as fluoride ion, extractable from the cured encapsulant when placed in contact with moisture. High levels of such ions are believed by many smart card manufacturers to increase the possibility of corrosion (due to the formation of acid by virtue of moisture absorption, such as from the atmosphere or through mishandling). These halide ions are present for the most part from the cationic photoinitiator counter ion.
Many known cationic photoinitiators have as a counter ion a phosphorous or antimony metal complex with the appropriate number of fluorine atoms per metal atom. While such photoinitiators are very effective to initiate photocure, the counter ions of these photoinitiators are loosely bound moieties, which are readily extractable. As such, these counter ions tend to leach out from reactions product of the curable composition in which they are used and/or hydrolyze to liberate halide ions under exposure to moisture, such as from the atmosphere or otherwise. In the context of surface mount electronic component attachment, see U.S. Pat. No. 4,916,805 (Ellrich), which discloses certain photoinitiators having counter ions, such as PF6−, BF4−, AsF6−and SbF6−.
Rhodia Chemie make available commercially a cationic photoinitiator for silicone-based release coatings, whose counter ion contains fluoride atoms covalently bound to aromatic carbon atoms of the counter ion, such as B(C6F5)4. See International Patent Application Nos. PCT/FR97/00566 and PCT/FR98/00741. See also Rhone-Poulenc Chemie's U.S. Pat. No. 5,550,265 (Castellanos), U.S. Pat. No. 5,668,192 (Castellanos), U.S. Pat. No. 6,147,184 (Castellanos), and U.S. Pat. No. 6,153,661 (Castellanos).
However, Rhodia Chemie (or Rhone-Poulenc Chemie) has not taught, suggested or promoted such photoinitiators for use in photocurable adhesive or encapsulant compositions, particularly those based on epoxies, or recognized the low level of fluoride ion that is extractable from reaction products formed by initiation through such cationic photoinitiators. Rhodia Chemie has reported low levels of hexane extractable silicone from reaction products. See S. R. Kerr, III, “Next Generation UV Silicone Release Coatings”, Adh. Age, p. 26-34 (August 1996).
Another drawback to known photocurable smart card encapsulants is the wavelength at which cure is designed to occur. That is, the memory device (or, EPROM) often used in smart card construction is particularly sensitive to wavelengths at 254 nm. At this wavelength, the EPROM may become erased. Therefore, current photocurable compositions that have been offered for sale as smart card encapsulants, but which cure at such wavelengths, are of limited utility.
Accordingly, the need exists for a photocurable composition, particularly well-suited for electronic component encapsulation applications, whose reaction products have low levels of extractable halide ion, and which may be designed to be curable at wavelengths greater than about 290 nm.